The present invention relates to a process for the preparation of an additive modified thermoplastic. More particularly, the present invention relate to a process for preparing a concentrated additive modified thermoplastic polycarbonate resin, aromatic polyester carbonate resin or mixture of polycarbonate and aromatic polyester carbonate resin and thereafter blending this concentrate with a thermoplastic resin to prepare the desired resin blend.
Manufacturers of molded parts from thermoplastic resins are frequently called upon to prepare objects utilizing a wide variety of different additive modified thermoplastic polymers. It has been frequently the practice in the past for resin suppliers to prepare and supply completely formulated resin blends meeting the desired end use applications of the molder. Recent experience has indicated that a more efficient technique is to supply smaller quantities of concentrated resin blends (referred to as a "concentrate") to a party desiring to prepare an additive modified blend and thereafter the desired additive containing resin may be prepared by mixing or "letting down" the concentrate with quantities of an unmodified thermoplastic resin (let down resin). With this technique it has been possible to realize significant economies in raw material usage, working capital, transportation, storage, and other costs as well as to realize increased flexibility in the supply of desired additive modified blends.
In the preparation of additive modified thermoplastics incorporating polycarbonate resins, aromatic polyester carbonate resins, mixtures of polycarbonate and aromatic polyester carbonate resins, and mixtures of the foregoing with other resins such as polyesters, polyphenylene oxide, ABS resins, etc., it has been discovered that often the resulting properties of the blend prepared by an on-site blending of an additive containing concentrate and a let-down resin are adversely affected. In particular, it has been discovered that impact properties as measured by Izod impact, Gardener impact, Dart drop or other instrumented impact measurement may be deleteriously affected. For example, in one particular embodiment of the present invention, it is desirable to combine the additive concentrate and the let-down resin in a molding machine or other suitable blending device immediately prior to the molding of objects therefrom. This operation is known as "direct molding". However, available equipment is designed to melt the resin and not necessarily obtain thorough mixing and thus direct molding often fails to obtain complete and adequate dispersion of the let-down resin and the additive concentrate. It has now been discovered that the resulting molded part prepared by direct molding often does not achieve impact properties as high as would be desired. Addition of a mixing nozzle or similar device may overcome some of these difficulties, however, a continued need for improvement still exists.
A second application for the present invention concerns the utilization of scrap resins obtained by trimming of excess resin from molded parts or by recycling defective plastic parts (referred to in the industry as "regrind" resin). In order to improve efficiency, it is highly desirable to reincorporate such regrind into the molten extrudate used in the molding process. Effective blending of such regrind resins which may include paint and other contaminants is often not possible utilizing the molding machines or even extruders or other mixing equipment presently available. As a result, molded objects incorporating regrind often fail to exhibit physical properties, particularly impact properties, as high as are obtained b objects prepared from virgin resins.
A final problem in the industry occurs in the preparation of the concentrate resins. Resins containing high levels of additives often are deficient in desired properties. For example, in the melt processing of such concentrates "surging" which is a variation in the amount of molten resin passing from the extruder or other malaxing device may be encountered. Also, "strand drops" or slubs may form in the strands of molten thermoplastic to be pelletized or otherwise comminuted.
It would be desirable if there were provided a technique allowing for the facile preparation of additive modified thermoplastic polymers by blending concentrates of polycarbonate resins, aromatic polyester carbonate resins or mixtures thereof with a let-down resin which overcomes the above stated deficiencies.
It would further be desirable to provide a process which permits the incorporation of regrind into a molding resin without resulting in a loss of physical properties in molded objects prepared therefrom.